let mut vec = Vec:: new (); for n in 0..10000 { let new_doc = doc! { "title" : "T" , "year" : 2020, "plot" : "plot description" , }; vec.push(new_doc); } all.insert_many(vec, None).unwrap(); // drop collection let pipeline = vec![ doc! { "$documents" : [ { "dens" : 0 } ] }, doc! { "$densify" : { "field" : "dens" , "range" : { "step" : 1, "bounds" : [0, 10000]} }}, doc! { "$addFields" : doc! { "title" : "T" , "year" : 2020, // "rand" : { "$rand" : {} }, "_id" : { "$rand" : {} }, }}, // doc! { "$out" : "all" }, doc! { "$merge" : "all" }, ]; db.aggregate(pipeline, None).unwrap();